In the prior art, there have been a number of proposals of retroreflective articles having excellent wide-angle performance, with improved entrance angle characteristics, observation angle characteristics, and rotation angle characteristics.
With respect to improvement of the entrance angle characteristics or observation angle characteristics of such cube-corner type retroreflective sheets and retroreflective articles, and in particular of triangular-pyramidal cube-corner retroreflective sheets and retroreflective articles, various proposals have been made from long ago, and various improvements have been studied. Many of these technologies improve the entrance angle characteristics by inclining the optical axes of some of the retroreflective elements.
For example, in U.S. Pat. No. 2,310,790 by Jungersen, an arrangement of retroreflective elements in various shapes on a thin sheet is described. The triangular-pyramidal reflective elements given as examples in this U.S. Patent include regular-triangular triangular-pyramidal reflective elements, the vertices of which are positioned at the centers of the base triangles, and which have no inclination of the optical axis, as well as isosceles-triangular triangular-pyramidal elements, the vertices of which are not positioned at the centers of the base triangles, and it is stated that light is caused to be reflected effectively (entrance angle characteristics are improved) for approaching vehicles.
Further, it is stated as the size that the depth of the triangular-pyramidal reflecting elements is within 1/10 inch (2,540 μm). And, in FIG. 15 of this U.S. Patent, a triangular-pyramidal reflecting element pair is shown the optical axis of which is inclined in the positive (+) direction; the angle of inclination (θ) of this optical axis, determined from the ratio of the long edges to the short edge in the base isosceles triangle of the triangular-pyramidal reflecting elements shown in the figure, is estimated to be approximately 6.5°.
Further, in European Patent No. 137,736B1 by Hoopman, a retroreflective sheet and retroreflective article are described in which pairs of inclined triangular-pyramidal cube-corner retroreflective elements, the triangular shapes in the base plane on a thin sheet of which are isosceles triangles, rotated 180° with respect to each other, are arranged with base faces on a common surface in closest-packed fashion. The optical axis of the triangular-pyramidal cube-corner retroreflective elements described in this patent are inclined in what is the negative (−) direction in this Specification, and the angle of inclination is shown to be approximately 7° to 13°.
Further, in U.S. Pat. No. 5,138,488 by Szczech, a retroreflective sheet and retroreflective article are disclosed, in which inclined triangular-pyramidal cube-corner retroreflective elements, the triangular shape in the base plane on a thin sheet of which are isosceles triangles, are similarly arranged with base faces on a common surface in closest-packed fashion. In this U.S. Patent, the optical axis of the triangular-pyramidal reflective elements is inclined in the direction common to two triangular-pyramidal reflective elements facing each other and forming a pair, that is, in the positive (+) direction explained below, the inclination angle is approximately 2° to 5°, and the element size is stipulated to be from 25 μm to 100 μm.
Further, in European Patent No. 548,280B1 corresponding to the above patent, it is stated that the direction of inclination of the optical axis comprises the common edge of two elements forming a pair, and that the distance of the vertex of an element and the plane perpendicular to the common plane is equal to the distance between the point at which the element optical axis intersects the common plane and the perpendicular plane, and that the inclination angle is approximately 2° to 5°, and the element size is 25 μm to 100 μm.
In this way, in the European Patent No. 548,280B1 of Szczech, the optical axis is in a range from approximately 2° to 5°, comprising both positive (+) and negative (−) directions. However, in the embodiments of the above U.S. Patent and European Patent of Szczech, only triangular-pyramidal reflective elements having an optical axis inclination angle of (−)8.20, (−)9.2°, and (−)4.3°, and with an element height (h) of 87.5 μm, are disclosed.
In addition, various proposals have been made relating to improvement of observation angle characteristics.
In U.S. Pat. No. 4,775,219 of Appeldorn, V-shape grooves forming elements present an asymmetrical shape such as shown in FIG. 17 which explains the present invention, and have a slight deviation with respect to the angle of the theoretical V-shape grooves forming cube corners. Further, by periodically modifying the deviation which imparts asymmetry to adjacent V-shape grooves, an attempt is made to improve observation angle characteristics.
However, periodic modification of the angle of adjacent V-shape grooves increases the difficulty of die machining. Even if this difficulty could be overcome, the combination of deviations which could be imparted is limited, and a uniform reflected light spreading could not be imparted. Further, it has been necessary to prepare several types of diamond bits to form V-shape grooves and other machining tools for one V-shape groove direction. And, high-precision machining technology has also been necessary when forming asymmetric V-shape grooves.
Further, in U.S. Pat. No. 5,171,624 by Walter, a triangular-pyramidal retroreflective element is disclosed, the reflective faces of which are formed using a machining tool having a curve-shape cross-sectional shape, having a fixed quadratic surface cross-sectional shape. In a triangular-pyramidal retroreflective element in which reflective side faces having a quadratic surface are formed, appropriate dispersion of retroreflective light is possible, and improvement of observation angle characteristics is obtained.
However, it is extremely difficult to fabricate machining tools having such curved-surface cross-sectional shapes by design. Hence due to the difficulty of machining such tools, it has been very difficult in the past to obtain quadratic surfaces by design. And, it has not been possible to form quadratic surfaces in various shapes, determined only by the shapes of machining tools used, on the same retroreflective article in given surface shapes.
In U.S. Pat. No. 5,565,151 by Nilsen, a portion of a reflecting side face (A-B-H) is cut away, and an attempt is made to promote the dispersion of retroreflective light and improve observation angle characteristics by means of the triangular-columnar shapes (A-A1-A2-B2-B1-B) formed thereby and the new reflecting side faces (A2-H1-B2).
However, in the invention of Nilsen there is no description of the kind of triangular columnar shape which is preferable for installation, nor of the kind of angle at which new reflecting side faces are to be formed. Further, special tools are required to cut away a portion of reflecting side faces and form a portion of the triangular columnar shape. And, elements with the newly formed triangular columnar shape do not have retroreflection functions, but are simply designed to obtain spreading of retroreflective light through dispersion of light in various directions.
However, in the above-described technology to improve entrance angle characteristics and observation angle characteristics, there is no attainment of improved rotation angle characteristics.
Various proposals relating to improvement of rotation angle characteristics have been made; in all such proposals, an attempt is made to improve rotation angle characteristics by partitioning the directions of retroreflective elements, the base faces of which have triangular shapes, into various regions and combining regions.
Examples include U.S. Pat. No. 5,022,739, U.S. Pat. No. 5,132,841, and U.S. Pat. No. 5,175,645, by Bennett et al; U.S. Pat. No. 6,036,322, by Nilsen et al; U.S. Pat. No. 5,706,132 and U.S. Pat. No. 5,936,770, by Nestegard et al; and U.S. Pat. No. 5,898,523, by Smith.
However, although the shapes into which element regions are partitioned and the directions of triangular-pyramidal cube-corner elements are different, all of the technologies of these patents can be said to employ the same basic technique.